Information fraud is a serious problem for banks, businesses, and consumers. Fast personal identification is verified by the use of photographic identification cards, credit cards, and other cards. However, with the rapid advances in image processing technology such as scanners, printers, and copiers, it has become increasingly simple to reproduce authentic looking fingerprints, pictures, and patterns. Techniques such as the embossed hologram on credit cards are no longer a reliable solution to the problem because they can be easily copied. There is therefore a need to develop more technically sophisticated systems to handle the counterfeiting problem.
There has been a surge in interest in biometrics (for example, unique signatures such as facial features, fingerprints, retina, voice patterns, and DNA) as part of a security system to protect sensitive information such as financial transactions. Also, in light of the present ease of circumventing security measures, such as passwords, in order to access computer information, computer and financial companies have a strong interest in implementing some form of biometric protection device to secure information held by personal computers. To counteract these fraudulent practices, all-optical correlator systems have been developed. For example, an all-optical spatial correlation of two phase-encoded images in a photochromic material in a four-wave mixing configuration may be utilized to inspect and verify biometrics. In particular, one phase-encoded biometric may be affixed to a product such as an identification card. The other phase biometric is stored in the security system for comparison with the object biometric. This biometric is then verified by the optical correlator that incorporates a holographic filter in order to verify authenticity.
Complex phase/amplitude patterns that cannot be seen and cannot be copied by an intensity-sensitive detector such as a charge-coupled device (CCD) camera are utilized for verification of the authenticity of items bearing the holography. Biometrics can be imaged on thin plastic materials using embossing techniques such as those used to imprint the holograms found on many cards. The code in the biometric is known only to the authorized producer of the card. The phase portion of the pattern consists of a two dimensional holography, which is invisible under ordinary light. The biometric cannot be analyzed by looking at the card under a microscope, photographing it, or reading it with a computer scanner in an attempt to reproduce it. However, a combination of performance limitations and the high cost of existing optical materials has limited the application of all-optical correlator systems.
Several types of materials have been investigated for holographic all-optical correlators. Among these are inorganic photorefractive crystals, photorefractive polymers, and photodynamic proteins, such as bacteriorhodopsin. Inorganic photorefractive crystals, such as BaTiO.sub.3, KNbO.sub.3, and LiNbO.sub.3, have been investigated, such as described by White et al., Phys. Lett., Vol. 37, pages 5-7 (1980), but their difficulty of processing and high cost have limited their widespread use. Photorefractive polymers have received attention because they can be made into thin films, and after poling exhibit photorefractive properties, such as described by Moerner et al., J. Opt. Soc. Am., Vol. 11, pages 320-330 (1994); Meerholz et al., Nature, Vol. 371, pages 497-500 (1994); Volodin et al., Nature, Vol. 383, pages 58-60 (1996); Kippelen et al., Proc. SPIE, Vol. 3144, pages 176-184 (1997); and Wada et al., Proc. SPIE, Vol. 3144, pages 186-194 (1997). For example, all optical encoding of documents has been tried using a holographic filter based on a photorefractive polymer [2,5-dimethyl-4-(p-nitrophenylazo)anisole; poly(N-vinylcarbazole); N-ethylcarbazole; 2,4,7-trinitro-9-fluorenone], as described by the above-referenced publications by Meerholz et al., and Volodin et al. Problems related to chromophore crystallization and phase separation were responsible for the early failure of these types of polymers. To increase stability, modifications of the composite polymer were tried, but they caused a reduction in the diffraction efficiency of the holographic filter.
One of the drawbacks of photorefractive polymers has been the high electric field strengths needed in order to align the chromophores. In contrast, liquid crystals can be aligned using much lower fields, as described by Wiederrecht et al., Science, Vol. 270, pages 1794-1797 (1995), and Khoo et al., Opt. Lett., Vol. 19, pages 1723-1725 (1994). The majority of liquid crystals are based on the twisted nematic (TN), supertwisted nematic (STN), and active matrix (TFT) models. With an electric field applied, the molecules reorient parallel to the normal. However, there is a finite switching speed which may cause problems when fast switching is required. The response time for a typical STN device is about 200 milliseconds and for a TN device is about 30 to 50 milliseconds. Another technique to reduce switching time is the use of ferroelectric smectic liquid crystals (FSLC), as described, for example, by Lacey, "Liquid Crystals and Devices," in Introduction to Molecular Electronics, Petty et al. (eds.), Oxford University Press, New York, pages 185-219 (1995), for fast switching LC materials for display applications. Ferroelectric liquid crystals are being developed to provide fast switching LC displays, fast enough for television and other real-time applications. In smectic liquid crystals, there is a tendency for molecules to cluster into separate planar sheets. A special case of these is the class of cholesteric liquid crystals where the directrix lies in the plane of the ordered layers but rotates from layer to layer to trace out a helix. Biopolymers belonging to this class may effectively compete with inorganic, organic, and polymeric optical materials particularly with respect to cost, environmental stability, and performance.
Biological molecules may be used to fabricate devices with smaller sizes and faster data handling capabilities than currently available semiconductor devices. Proteins in particular have proven to be well suited for applications in optical information processing and all-optical computing. Photochromic materials such as bacteriorhodopsin (bR) have been actively investigated with a view to a number of applications including holography, optical RAM (random access memory), and optical information processing. Bacteriorhodopsin and photopolymers containing bR are well suited to the development of real-time holographic materials for optical information processing, all-optical computing, and other applications in photonics. For example, a bR-based all-optical correlator using the bR.sub.96N mutant in a Fourier optical architectural scheme that implemented spatial frequency filtering on an input image was described by Thoma et al. in Opt. Lett., Vol. 16, pages 651-653 (1991), in Opt. Lett., Vol. 17, pages 1158-1163 (1992), and in Opt. Eng., Vol. 34, pages 1345-1351 (1995). Owing to the fact that proteins such as bacteriorhodopsin are typically more sensitive to light than inorganic crystals, a molecular switch may be realized with very low light levels.
Although bacteriorhodopsin has shown promise as a holographic material, its use has been limited by low diffraction efficiency. An optical element, such as a holographic grating or a holographic filter, that retains the desirable properties of the photoresponsive compound, such as photochromic bacteriorhodopsin, while overcoming limitations such as low diffraction efficiency and stability against phase separation and crystallization, would be advantageous for optical correlator systems, dynamic holographic recording systems, holographic gratings and filters, optical switches, and other related optical, electro-optic, and thermo-optic systems.